The present invention relates to the inspection of guide plates within the control rod guide tubes of nuclear reactors and more particularly to a delivery tool for positioning a CCTV camera above one of a plurality of holes in a respective guide plate to be inspected.
The core of a modern nuclear reactor of the type used to generate electrical power generally includes an upper internals assembly disposed over a lower core barrel. The lower core barrel houses an array of fuel rod assemblies which generate heat as a result of a controlled fission reaction that occurs in the uranium oxide pellets present in their individual fuel rods. Water is constantly circulated from the lower core barrel through the upper internals and out through outlet nozzles provided in the walls of an upper core barrel in order to transfer the heat generated by the fuel rod assemblies to heat exchangers which ultimately convert this heat into usable, nonradioactive steam.
The rate of the fission reaction taking place within the fuel rod assemblies is regulated by means of a control rod assembly. Each of these control rod assemblies is formed from an array of stainless steel tubes each containing a neutron absorbing substance such as silver, indium or cadmium. These stainless steel tubes (known as "rodlets" in the art) are suspended from a spider-like bracket. A reciprocable drive rod is connected to the spider-like bracket for either inserting or withdrawing the rodlets of the control rod assembly deeper into or farther out of each of the fuel rod assemblies in order to modulate the amount of heat generated thereby.
The upper internals assembly includes an upper core barrel arranged in tandem with the lower core barrel of the reactor. The ceiling of the upper core barrel is formed from an upper support plate. The peripheral edge of this support plate is seated around the upper edge of the upper core barrel. Both the support plate and the upper core plate which underlies it include a plurality of apertures for both conducting the stream of hot, pressurized water exiting the fuel rod assemblies to the heat exchangers, as well as for conducting a control rod assembly either into or out of each of the fuel rod assemblies. Separate guide tubes are provided between apertures in both the support and core barrel plates which are aligned with each other and with one of the fuel assemblies in the lower core barrel. The purpose of these guide tubes is to align and guide the relatively long and flexible rodlets of the control rod assemblies into a particular fuel assembly. Additionally, separate support columns are connected between the upper support plate and the underlying upper core plate in order to suspend and support the upper core plate over the fuel rod assemblies contained within the lower core barrel.
Applicants have observed that the control rods used in some nuclear reactors are manifesting noticeable mechanical degradation in the form of wear, which in some cases occurs after only a few fuel cycles. The applicants have further observed that this degradation is caused by stepping and fretting wear between the control rods and the guide plates which guide the individual control rods into the reactor core.
Each guide tube encloses a number of guide plates which are perpendicularly positioned with respect to the longitudinal axis of the guide tube. Each of the guide plates have an array of through holes approximately 0.42" (1.07 cm) in diameter formed therein which receive the reciprocably movable control rods to guide them into and out of the reactor core. These guide plates are uniformly spaced along the axis of the guide tube and are 10.57 inches (26.85 cm) apart in the upper portion of the guide tube and 11.57 inches (29.39 cm) apart in the lower portion thereof. Further, each guide plate include a central bore 2.3 inches (5.84 cm) in diameter which presents the only access to the individual guide plates. There can be up to twelve guide plates in a given guide tube, with each guide plate containing up to twenty-four holes.
Because of the premature wearing of the control rods in some instances, it has become desirable to obtain wear characteristics of the holes of each individual guide plate so that this data may be correlated with the control rod wear scars. This would enable a model of the wear mechanism to be developed by maintenance technicians in order to solve the wear problem associated with the control rods and guide plates. Consequently, there is a pressing need for an inspection system for inspecting each of the individual holes in the guide plates and transforming this data into a usable form.
Devices known at this time for inspecting the interior of various tubular members are directed toward inspecting the inner wall of such members and not the inspection of plates enclosed within such tubular members. Examples of such inspection assemblies are set forth in U.S. Pat. No. 4,255,762 to Takeyasu et al. and U.S. Pat. No. 4,424,531 to Elter et al. Each of these assemblies employs a camera for transmitting images of the wall of the inspected tube to an outside control center, and may include sensors for determining the relative positioning of the assembly within the tube. However, it is not possible for these assemblies to inspect holes in the surface of a diametrically positioned plate within tube.
Similarly, the remote inspection assembly disclosed in German Democratic Republic (East German) Patent No. 208,884 is capable of inspecting the interior of nuclear reactor pressure vessels. While this assembly includes a monitor and TV camera for recording images, it is not capable of inspecting holes in the surface of transversely positioned plates within an elongated tube.